(1) Field of the Invention
The present invention relates to antivibration suspension means for a tie bar of a main power transmission gearbox, to an antivibration suspension device including the suspension means, and to an aircraft having such a device.
The invention is thus situated in the narrow technical field of devices for reducing aircraft vibration.
(2) Description of Related Art
Aircraft include in particular rotorcraft having at least one lift rotor connected to a carrier structure, the carrier structure conventionally being referred to as an “airframe” or a “fuselage”.
Such an aircraft also includes a power plant for driving a power transmission gearbox secured to the carrier structure of the aircraft. The gearbox then includes a mast for driving the lift rotor in rotation.
It should be observed that the gearbox is often connected to the carrier structure via a bottom wall and auxiliary fastener means generally comprising three or four sloping tie bars. Because of its shape, the person skilled in the art sometimes refers to the structure for mounting the lift rotor and including the gearbox and the tie bars as a “pylon”.
The lift rotor and/or the gearbox may give rise to vibration that is uncomfortable for the occupants of the aircraft because of the vibratory motion and the noise that are generated in the aircraft by that vibration. Furthermore, equipment of the aircraft arranged in the carrier structure can be damaged by the vibration generated by the mechanical assembly comprising the gearbox and the lift rotor.
Under such circumstances, various suspension devices are known for suspending the gearbox in order at least to reduce vibration within the aircraft, and in particular within a cabin, for reasons of pilot and passenger comfort.
Such a suspension device must be capable firstly of transmitting the static load induced by the mechanical assembly, and secondly of filtering out the vibration induced by the mechanical assembly. It is observed that this vibration is produced at very low frequency and is then uncomfortable for the occupants of the aircraft.
Furthermore, it can be understood that a manufacturer seeks to obtain a suspension device that is of minimum impact in terms of weight and cost.
It should be observed that certain aircraft include a suspension rotor that may operate in flight at different speeds of rotation. It is then advantageous to have a suspension device that is adapted to that type of technology.
In the state of the art, document FR 2 363 737 discloses a vibration isolation device that connects a vibrating mass to a body.
The isolation device comprises a lever arm loaded by a weight, with pivot means connecting said vibrating weight to said arm. In addition, a torsion spring device is connected to the lever arm.
Document FR 2 474 996 discloses a suspension device for a mechanical assembly of a helicopter.
That suspension device presents three sloping tie bars and a suspension plate of a power transmission gearbox. The suspension plate has a central portion secured to a bottom of the gearbox and one arm per sloping tie bar. Each arm is hinged in the region of its proximal end firstly to the fuselage and secondly to an extreme zone of the corresponding tie bar. The distal end of each arm also carries a flyweight.
Document FR 2 499 505 discloses a suspension device having at least three tie bars fastened to a power transmission gearbox via their top ends.
Under such circumstances, the bottom ends of the tie bars are hinged to arms arranged radially around a base of the gearbox to which the arms are connected. Each arm is also hinged to a strong point of the aircraft fuselage and carries a weight mounted at the end of a support.
The base of the gearbox is also arranged on a suspension plate comprising a diaphragm that is deformable in bending.
According to document FR 2 747 098, a suspension device has flyweights and means for moving those flyweights in a longitudinal direction along a support.
Under such circumstances, the device includes at least one sensor capable of measuring the values of at least one parameter representative of a performance criterion of the device, and a computer receiving said values in order to servo-control the positions of the flyweights via the means for moving them.
That device thus makes it possible to combat vibration that occurs at varying frequency.
Document FR 2 795 386 discloses a suspension device having a plurality of sloping tie bars.
Each sloping tie bar is hinged to a carrier structure via a lever supporting a flyweight. Each lever is then connected to the carrier structure by a torsion spring.
Document FR 2 787 762 discloses a suspension device having a plurality of sloping tie bars.
Each sloping tie bar is hinged to a carrier structure by a lever supporting a flyweight. Each lever is connected to the carrier structure via a pivot connection, and to a power transmission gearbox via a torsion spring.
Reference may also be made to document WO 03/100284.